1. Field of the Invention
The present invention relates to an apparatus and method for estimating the amount of NOx adsorption in a NOx trap catalyst provided in an exhaust passage.
2. Description of the Related Art
Conventionally, a NOx trap catalyst adsorbs NOx (nitrogen oxides) in exhaust gases when the exhaust air-fuel ratio is lean, and releases and reduces the adsorbed NOx when the exhaust air-fuel ratio is rich. Specifically, the catalyst adsorbs NOx in exhaust gases as nitrate salt in an oxygen excess condition (oxidized atmosphere) and reduces the adsorbed NOx into nitrogen in a carbon monoxide excess condition (reduced atmosphere).
In an engine equipped with the above catalyst, rich spike is performed such that the mode of operation is intermittently switched to rich operation before the NOx adsorption amount reaches a level of saturation, thereby suppressing the deterioration of catalytic performance caused by an increase in NOx adsorption amount. As a result, the catalyst is regenerated, and exhaust gases are satisfactorily purified.
The rich spike can be implemented in two ways: in-cylinder rich operation and out-of-cylinder rich operation. Specifically, the in-cylinder rich operation can be implemented by, for example, a method in which exhaust-gas recirculation (EGR) is performed at a high recirculation rate such that carbon monoxide (CO) emitted from cylinders by imperfect combustion is used as a reductant, or a method in which unburned fuel (HC) as a reductant is supplied into cylinders by post (after) injection in exhaust strokes. On the other hand, the out-of-cylinder rich operation can be implemented by, for example, a method in which the HC is added to an exhaust passage, that is, fuel is directly supplied to the catalyst.
Here, to perform the rich spike, it is necessary to accurately estimate or detect the NOx adsorption amount. For this purpose, there has been proposed a technique in which the NOx adsorption amount is estimated using a mathematical catalytic model based on chemical/physical reactions of a catalyst (Japanese Laid-Open Patent Publication (Kokai) No. H09-72235).
According to this technique, at least one oxygen sensor is provided downstream of the catalyst so that the amount of NOx adsorption in the catalyst can be estimated. However, the principal objective of this technique is to control the air-fuel ratio for a three-way catalyst. That is, it is difficult to apply this technique to the NOx trap catalyst because the three-way catalyst differs from the NOx trap catalyst in model structure. Also, in the case where the NOx adsorption amount is estimated using other catalytic model, it is necessary to sequentially change model equations because the characteristic value varies depending on types of catalyst. Besides, to cope with the deterioration of the catalyst, more detailed modeling is required. Thus, according to the above conventional technique, even though it is possible to estimate the NOx adsorption amount, problems to be addressed still remain where accurate estimation of the NOx adsorption amount is concerned.
Also, to estimate the amount of NOx adsorption in the NOx trap catalyst, it is necessary to keep in mind that the NOx emission during the rich spike, i.e. a decrease in NOx adsorption amount has to found. This is because the latest NOx adsorption amount cannot be accurately identified unless a decrease in NOx adsorption amount caused by the rich spike is taken into consideration. To estimate the NOx adsorption amount, a method can be envisaged in which values related to a reductant emitted from cylinders are estimated in advance, the NOx emission is calculated using the values, and the NOx adsorption amount is then estimated. However, this method raises concern that the scope of application of the rich spike is narrowed.